1. Field of the Invention
The present invention relates to a structural steel product suitable fo use in constructions, bridges, ship constructions, marine structures, steel pipes, line pipes, etc. More particularly, the present invention relates to a weldable structural steel product which is manufactured using fine complex precipitates of TiN and MnS, dispersed in such a fashion that MnS surrounds TiN, thereby being capable of simultaneously exhibiting improved toughness and strength in a heat-affected zone. The present invention also relates to a method for manufacturing the weldable structural steel product, and a welded construction using the welded structural steel product.
2. Description of the Prior Art
Recently, as the height or size of buildings and other structures has increased, steel products having an increased size have been increasingly used. That is, thick steel products have been increasingly used. In order to weld such thick steel products, it is necessary to use a welding process with a high efficiency. For welding techniques for thick steel products, a heat-input submerged welding process enabling a single pass welding, and an electro-welding process have been widely used. The heat-input welding process enabling a single pass welding is also applied to ship constructions and bridges requiring welding of steel plates having a thickness of 25 mm or more. Generally, it is possible to reduce the number of welding passes at a higher amount of heat input because the amount of welded metal deposited is increased. Accordingly, there may be an advantage in terms of welding efficiency where the heat-input welding process is applicable. That is, in the case of a welding process using an increased heat input, its application can be widened. Typically, the heat input used in welding is in the range of 100 to 200 kJ/cm. In order to weld steel plates further thickened to a thickness of 50 mm or more, it is necessary to use super-high heat input ranging from 200 kJ/cm to 500 kJ/cm.
Where high heat input is applied to a steel product, the heat affected zone, in particular, its portion arranged near a fusion boundary, is heated to a temperature approximate to a melting point of the steel product by welding heat input. As a result, growth of grains occurs at the heat affected zone, so that a coarsened grain structure is formed. Furthermore, when the steel product is subjected to a cooling process, fine structures having degraded toughness, such as bainite and martensite, may be formed. Thus, the heat affected zone may be a site exhibiting degraded toughness.
In order to secure a desired stability of such a welding structure, it is necessary to suppress the growth of austenite grains at the heat affected zone, so as to allow the welding structure to maintain a fine structure. Known as means for meeting this requirement are techniques in which oxides stable at a high temperature or Ti-based carbon nitrides are appropriately dispersed in steels in order to delay growth of grains at the heat affected zone during a welding process. Such techniques are disclosed in Japanese Patent Laid-open Publication No. Hei. 12-226633, Hei. 11-140582, Hei. 10-298708, Hei. 10-298706, Hei. 9-194990, Hei. 9-324238, Hei. 8-60292, Sho. 60-245768, Hei. 5-186848, Sho. 58-31065, Sho. 61-79745, and Sho. 64-15320, and Journal of Japanese Welding Society, Vol. 52, No. 2, pp 49.
The technique disclosed in Japanese Patent Laid-open Publication No. Hei. 11-140582 is a representative one of techniques using precipitates of TiN. This technique has proposed structural steels exhibiting an impact toughness of about 200 J at 0° C. (in the case of a matrix, about 300 J). In accordance with this technique, the ratio of Ti/N is controlled to be 4 to 12, so as to form TiN precipitates having a grain size of 0.05 μm or less at a density of 5.8×103/mm2 to 8.1×104/mm2 while forming TiN precipitates having a grain size of 0.03 to 0.2 μm at a density of 3.9×103/mm2 to 6.2×104/mm2, thereby securing a desired toughness at the welding site. In accordance with this technique, however, both the matrix and the heat affected zone exhibit substantially low toughness where a heat-input welding process is applied. For example, the matrix and heat affected zone exhibit impact toughness of 320 J and 220 J at 0° C. Furthermore, since there is a considerable toughness difference between the matrix and heat affected zone, as much as about 100 J, it is difficult to secure a desired reliability for a steel construction obtained by subjecting thickened steel products to a welding process using super-high heat input. Moreover, in order to obtain desired TiN precipitates, the technique involves a process of heating a slab at a temperature of 1,050° C. or more, quenching the heated slab, and again heating the quenched slab for a subsequent hot rolling process. Due to such a double heat treatment, an increase in the manufacturing costs occurs.
Japanese Patent Laid-Qpen Publication No. Hei. 9-194990 discloses a technique in which the ratio between Al and O in low steel (N≦0.005%) is controlled to be within a range of 0.3 to 1.5 (0.3≦Al/O≦1.5) in order to form a complex oxide containing Al, Mn, and Si. However, the steel product according to this technique exhibits a degraded toughness because when a welding process using a high heat input of about 100 kJ/cm, the transition temperature at the heat affected zone corresponds to a level of about −50. Also, Japanese Patent Laid-open Publication No. Hei. 10-298708 discloses a technique in which complex precipitates of MgO and TiN are utilized. However, the steel product according to this technique exhibits a degraded toughness in that when a welding process uses a high heat input of about 100 kJ/cm, the impact toughness at 0° C. in the heat affected zone corresponds to 130 J.
There have been many techniques for improving the toughness of the heat affected zone using TiN precipitates and Al-based oxides or MgO where a welding process using a high heat input is applied. However, there is no technique capable of remarkably improving the toughness of the heat affected zone where a welding process using a super-high heat input is carried out for a prolonged period of time at 1,350° C. or more.